This invention relates to xerographic printing systems and, more particularly, to improved apparatus for transferring toner from a photoreceptive drum to a paper web.
In xerographic copying and printing systems a drum having a photoreceptive surface is electrostatically charged with an image of the document to be reproduced. An electrostatic toner is distributed on the drum and arranged by the electrostatic charge to form a physical image corresponding to the document image. In the systems in which this invention finds special utility, the toner is then transferred from the photoreceptive surface of the drum to a paper web. To complete this transfer, the electrostatic attraction of the toner to the drum must be overcome.
The well known technique for accomplishing the toner transfer consists of creating an electrostatic charge on the side of the paper opposite from the drum having a polarity and at such a level that the toner particles are attracted away from the drum and onto the paper. Heretofore, this field has been produced by a corotron by means of a corona discharge. Such a system has several disadvantages including the need for a very high voltage supply, nonuniformities in the electrostatic field produced by the corona, ozone generation and significant manufacturing and service cost. Thus, there exists a need for a simpler and more effective transfer system.
The present invention accomplishes the toner transfer through the use of a flexible electrode which is in contact with the paper web and in very close proximity to the photoreceptive drum. The electrode establishes an electrostatic field which attracts the toner particles to achieve the transfer from the drum to the paper. This system uses a lower voltage supply, has a uniform electrostatic field, generates no ozone and is easy to fabricate and maintain.
Flexible electrodes can also be spaced near the leading and trailing edges of the contact zone between the paper and the photoreceptive drum to control the electrostatic fields present in these regions. These electrodes minimize the electrostatic field and related charge exchange which takes place when the paper makes and breaks contact with the drum.